The pthread_mutexattr_gettype() and pthread_mutexattr_settype() functions respectively get and set the mutex type attribute. This attribute
is set in the type parameter to these functions. The default value of the type attribute is PTHREAD_MUTEX_DEFAULT.
The type of mutex is contained in the type attribute of the mutex attributes. Valid mutex types include:
PTHREAD_MUTEX_NORMAL
This type of mutex does not detect deadlock. A thread
attempting to relock this mutex without first unlocking it will deadlock. Attempting to unlock a mutex locked by a different thread results in undefined behavior. Attempting to unlock an unlocked mutex
results in undefined behavior.
PTHREAD_MUTEX_ERRORCHECK
This type of mutex provides error checking. A thread attempting to relock this mutex without
first unlocking it will return with an error. A thread attempting to unlock a mutex that another thread has locked will return with an error. A thread attempting to unlock an unlocked mutex will return
with an error.
PTHREAD_MUTEX_RECURSIVE
A thread attempting to relock this mutex without first unlocking it will succeed in locking the
mutex. The relocking deadlock that can occur with mutexes of type PTHREAD_MUTEX_NORMAL cannot occur with this type of mutex. Multiple locks of this mutex require
the same number of unlocks to release the mutex before another thread can acquire the mutex. A thread attempting to unlock a mutex that another thread has locked will return with an error. A thread attempting
to unlock an unlocked mutex will return with an error. This type of mutex is only supported for mutexes whose process shared attribute is PTHREAD_PROCESS_PRIVATE.
PTHREAD_MUTEX_DEFAULT
Attempting to recursively lock a mutex of this type results in undefined behavior. Attempting to
unlock a mutex of this type that was not locked by the calling thread results in undefined behavior. Attempting to unlock a mutex of this type that is not locked results in undefined behavior. An implementation
is allowed to map this mutex to one of the other mutex types.
Upon successful completion, the pthread_mutexattr_settype() function returns 0. Otherwise, an error number is returned to indicate the error.
Upon successful completion, the pthread_mutexattr_gettype() function returns 0 and stores the value of the type attribute of attr in the object referenced by the type parameter. Otherwise an error number is returned to indicate the error.
It is advised that an application should not use a PTHREAD_MUTEX_RECURSIVE mutex with condition variables PTHREAD_MUTEX_RECURSIVE because the implicit unlock performed for a pthread_cond_wait() or pthread_cond_timedwait() will not actually release the mutex (if it had been locked multiple
times). If this occurs, no other thread can satisfy the condition of the predicate.